Surface Chemistry and Non-Stoichiometry of Nd2NiO4+δ Epitaxial Thin Films with Different Orientation and Strain

The influence of lattice strain on non-stoichiometry and surface chemical composition was investigated for epitaxial Nd2NiO4+δ (NNO) films during annealing in ultra high vacuum (below 10 mbar) and temperatures of up to 700C. (100)and (001)-oriented films with tensile and compressive lattice strain along c-axis were fabricated using pulsed laser deposition method. A significant decrease in the c-lattice parameter during annealing was found by x-ray diffraction (XRD) for the tensile strained films. X-ray photoelectron spectroscopy (XPS) showed that Ni reduction during annealing takes place only in compressively strained films, indicating the lower content of oxygen interstitials. A lower interstitial content in the compressively strained NNO films is consistent with the smaller c-lattice parameter measured by XRD and the easier reducibility of Ni measured by XPS. Cation segregation and morphological changes were found only for the compressively strained film surfaces. These results show that lattice strain along the c-axis is an important parameter that can alter the surface chemistry, and thus the oxygen exchange kinetics, on Nd2NiO4+δ at elevated temperatures.